The invention relates generally to immersion lithography apparatus and methods, and particularly to systems and methods for containing immersion fluid within a gap between a projection system and an object to be exposed by the projection system.
Lithography exposure apparatus are commonly used to transfer images from a reticle onto a semiconductor wafer during semiconductor processing. A typical exposure apparatus includes an illumination source, a reticle stage assembly that positions a reticle containing one or more patterns, a projection system, a wafer stage assembly that positions a semiconductor wafer, and a measurement system that precisely monitors the positions of the reticle and the wafer. As is known, lithography exposure apparatus also can be used to form images on substrates other than semiconductor wafers, for example, glass or quartz substrates in order to form, for example, flat panel displays such as LCD displays.
Immersion lithography is a technique that can enhance the resolution of lithography exposure apparatus by permitting exposure to take place with a numeral aperture (NA) that is greater than the NA that can be achieved in conventional “dry” lithography exposure apparatus. By filling the space between the final optical element of the projection system and the resist-coated target (wafer or other substrate), immersion lithography permits exposure with light that would otherwise be internally reflected at an optic-air interface. Numerical apertures as high as the index of the immersion fluid (or of the resist or lens material, whichever is least) are possible in immersion lithography systems. Liquid immersion also increases the wafer depth-of-focus, that is, the tolerable error in the vertical position of the wafer, by the index of the immersion fluid compared to a dry system having the same numerical aperture. Immersion lithography thus has the potential to provide resolution enhancement equivalent to a shift from 248 nm to 193 nm without actually decreasing the exposure light wavelength. Thus, unlike a shift in the exposure light wavelength, the use of immersion would not require the development of new light sources, optical materials (for the illumination and projection systems) or coatings, and should allow the use of the same or similar resists as conventional “dry” lithography at the same wavelength. In an immersion system in which only the final optical element of the projection system and its housing and the wafer (and perhaps portions of the stage as well) are in contact with the immersion fluid, much of the technology and design developed for dry lithography can carry over directly to immersion lithography.
However, because the wafer (or other substrate) moves rapidly in a typical lithography system, the immersion fluid in the immersion area tends to be carried away from the immersion area. If the immersion fluid escapes from the immersion area, that fluid can interfere with operation of other components of the lithography system. One way of attempting to prevent the escape of the immersion fluid is to form an immersion fluid barrier around the immersion area by supplying a pressurized gas to the region adjacent to the immersion area. Examples of such systems are shown in WO2004/090634 (corresponding to US 2006/0028632A1), US 2004/0165159A1, US 2004/0207824A1 (issued as U.S. Pat. No. 6,952,253) and US 2005/0007569A1. The disclosures of US 2006/0028632A1, US 2004/0165159A1, US 2004/0207824A1, U.S. Pat. No. 6,952,253 and US 2005/0007569A1 are incorporated herein by reference in their entireties.